The present invention relates to a coextrusion process for the manufacture of composite flat films or sheets comprising at least two layers of different base materials which are heat plastified in extruders and then united in a specially designed adapter means to form one melt stream or strand. In its broader aspects, this invention also relates to the manufacture of stretched composite films. The present invention further relates to an apparatus for carrying out this process by means of a specially designed adapter.
Composite or multi-layered films of thermoplastic materials are manufactured in order to take advantage in one product of the advantageous properties of different plastic materials. In addition to conventional process methods which have been used for a number of years, such as, e.g., bonding, laminating and extrusion coating, the process of coextrusion has gained more and more importance in recent times.
As far as the practical application of coextrusion is concerned, a number of different criteria have to be taken into account, including not only those which relate to the process itself, but also those relating to basic problems, e.g., the problem of adhesion.
The latter phenomenon is of practically no importance or of only minor importance in those cases where materials of an identical or of similar nature are employed, e.g., polyethylene and polypropylene. If thermoplastic materials of different types are to be coextruded, however, it is almost impossible to avoid the use of adhesion promoters or of special adhesion-improving processes, e.g., corona or gas treatment. No doubt, the film production process is made technically more complicated and thus ultimately more expensive by these process steps. Therefore, attempts have been made to increase the bonding strengths of coextruded products by mechanically linking their surfaces with each other, in addition to the normal physico-chemical adhesion mechanisms. The technical realization of such attempts is based on the idea of shaping the layer surfaces to be linked with each other such that a zipper-like meshing would be obtained.
A process of this kind has been described in "Kunststoffe", Vol. 69, No. 2 (1979), pages 81 and 82. Therein, a multi-layered film tubing is produced by means of a special ring die being composed of a fixed outer die body and rotating, profiled die lips. A spirally wound toothing of the individual layers results from this combination. A disadvantage of this process is, however, that a complicated and thus expensive special die is required which cannot be used for any other purposes. Even simple measures belonging to everyday production routine, such as the changeover of the production program to greater film widths or an exchange of the die for qualitative reasons due a contamination of the old die, require the disposition of a further special die, which is technically complicated and thus leads to higher operating expenses.
Another disadvantage of this process is represented by the limited number of toothed configurations. Since the adhesiveness of the composite film is largely determined by these toothed configurations, as great a number of them as possible is desired. The circumferential slot length of the ring dies which from time-to-time become available and the technological possibilities of providing the die lips with toothed configurations, automatically set limits to an unlimited multiplication of the toothed configurations.
Most of the objections made against the above process also apply to the extruder head described in German Auslegeschrift No. 21 23 331 and to the process according to U.S. Pat. No. 3,444,031. Apart from some minor apparatus variations, here, too, the manufacture of multi-layered, mechanically anchored flat films is described. In both cases, specially designed slot dies are used. The toothed profile which is to be obtained on the finished composite film is predetermined by distributor elements inserted into the die body.
The mechanical anchoring of coextruded polymer layers is, inter alia, also described in German Auslegeschrift No. 19 48 442. For manufacturing multilayered flat films, there is used in this process an adapter means having a circular cross-section which is attached directly before the slot die and by means of which the melt streams are united. In a special embodiment, the lip of the adapter means where the melt streams meet can be toothed. From the two-layered polymer stream which has been united in this profiled form, a flat film is formed in the directly subsequent slot die. Here the disadvantage quite obviously lies in the incongruity between the sizes of the adapter means and the slot die. If one assumes sizes of the die inlet channels of between 20 and 30 mm, as they are conventionally used today, and compares them with the working widths of the usually employed slot dies of about 1,500 mm, it becomes obvious that the density of toothings which can be achieved at the adapter lip, the size of which corresponds to that of the die inlet channel, will hardly be sufficient to increase the adhesiveness of the finished flat film. Moreover, some of these few toothings get lost, since they lie within the bent edge areas which are lost when the edges are trimmed off.
In this connection, still another disadvantage of the above-described adapter means becomes apparent, namely, the circular shape of the melt channel. This shape has a very unfavorable influence on the uniformity of the thickness of the individual layers and of the final composite films. The more forming processes the melt layers have to undergo, the greater is the danger of undesired profile deteriorations of the finished product.